1999 Lincoln Navigator heater core

My wife just had our navigator serviced at sears for an oil change. The mechanic there said he believed that the heater core is leaking. When you turn on the ac or heater you can smell antifreeze coming through the vents, it fogged up the windshield. The engine doesn't overheat, and we don't have heat. I'm mechanically inclined, but I've never changed out a heater core. We don't have the $1,200 to get it fixed, since we just put in 4 new tie-rods at $800, and $250 for a new exhaust system in my truck. I'm mechanically inclined, is it hard to do?